FIELD OF THE INVENTION
The invention relates to a multi-phase, high temperature-resistant material being formed of an alloy based on an intermetallic compound of the .gamma.-TiAl type, in particular for use in heat engines such as internal combustion engines, gas turbines and aircraft engines.
The development of heat engines is moving increasingly to higher power at a structural size which remains the same as far as possible, so that heat stress on individual components is continually increased and therefore improved heat resistance as well as strength are increasingly demanded from the materials being employed.
In addition to numerous developments in the materials field, for example nickel-based alloys, alloys based on an intermetallic compound of the .gamma.-TiAl type have particularly gained increasing interest for such a use in heat engines, because of the high melting point coupled with low density. Numerous developments deal with the attempt to improve the mechanical properties of such high-temperature materials. In addition to the improvement of the mechanical properties, the resistance to corrosive attack at the high temperatures that are in use particularly plays a special part, for example the resistance to attack by hot combustion gases, gaseous chlorides and sulphur dioxide.
Furthermore, the service life at lower temperatures is limited by condensed alkali metal sulphates and alkaline earth metal sulphates, so that an exploitation of the potential strength of these materials which is present per se, is prevented. In other words, the use temperature that is actually achievable as viewed from the high-temperature strength, is reduced because of the restricted oxidation resistance.
It is well known that the oxidation resistance of the binary titanium/aluminum compounds is completely inadequate for the above-mentioned applications, since the oxidation rate lies several powers of ten above that of superalloys used today, and their oxide layers have a low adhesive strength, which leads to continuous corrosive wear. It is known that at temperatures above 900 C., compounds based on titanium aluminide with significant contents of chromium and vanadium admittedly show good oxidation resistance which is comparable with that of superalloys used today, but show a completely inadequate oxidation behavior at lower temperatures, which is comparable with that of binary titanium aluminides, such as .gamma.-TiAl.
In the same way, the mechanical properties of such compounds are completely inadequate for industrial applications. At low temperatures, they have virtually no ductility, and they possess an inadequate creep resistance or fatigue strength at higher temperatures.